Technologies of transmitting the data of electric power usage have been known. With the technologies, a plurality of nodes capable of communicating wirelessly measure the electric power usage and transmit the data of the electric power usage to a gateway by a multi-hop communication through the nodes (Japanese Laid-open Patent Publication No. 2009-188929, for example). In Japanese Laid-open Patent Publication No. 2009-188929, all of the nodes in the system measure the electric power usage at a particular clock time and transmit the measured value to a gateway through relay nodes. The gateway collects the electric power usage data and transmits the data to a collection server. The collection server then uses the collected electric power usage data. In the communication system disclosed in Japanese Laid-open Patent Publication No. 2009-188929, the nodes, the gateway, and the collection server have a synchronized clock time. The nodes measure the electric power usage at a certain clock time and transmit the measured value of the electric power usage to the collection server.
In wireless communication, if a plurality of communications are performed at the same time on the same channel, collisions are generated and thus communications are unavailable in some communication methods. To cope with this, conventional technologies have been known in which the timing of communicating by the nodes is determined by the remainder acquired by dividing the value unique to the device of each of the nodes by a fixed value, for the purpose of distributing the timing of communicating by the nodes (Japanese Patent No. 3288162, for example).
In the communication system disclosed in Japanese Laid-open Patent Publication No. 2009-188929, the nodes transmit the electric power usage data to the gateway at a certain clock time. If a wireless communication system with a low transmission rate is used, the possibility of collisions during the wireless communications increases. This may reduce the collectability of the data. In an ad hoc network, in particular, the nodes indirectly communicate with the gateway through a plurality of nodes. If the nodes communicate with the gateway, therefore, the possibility of signal collisions on the wireless network increases according to the number of hops in addition to the number of nodes counted from a first-transmitting node to the gateway.
With the method disclosed in Japanese Patent No. 3288162, distributing the times of communications is determined by the remainder acquired by dividing the value unique to the device of each of the nodes by a fixed value. The unique value is, however, determined when the nodes are provided in the network, which can be hardly changed once the nodes are provided. The timing of transmitting the electric power usage data by the nodes can therefore not be controlled.